WO2011092278A1

WO2011092278A1 - Epoxy-based primer compositions

Info

Publication number: WO2011092278A1
Application number: PCT/EP2011/051194
Authority: WO
Inventors: Søren KIIL; Per Aggerholm SØRENSEN; Kim Dam-Johansen; Claus Erik Weinell; Erik KALLESØE
Original assignee: Hempel A/S; The Technical University Of Denmark
Priority date: 2010-01-29
Filing date: 2011-01-28
Publication date: 2011-08-04
Also published as: EP2528979A1

Abstract

The present application discloses a liquid epoxy-based primer composition comprising one or more free-radical scavengers in an amount of 0.5-20% by weight, said free-radical scavengers being selected from phenols and phenol multimers each substituted with electron-donating substituents, and application thereof by spraying. The application also discloses a coating system on a metal surface, comprising a coat of the epoxy-based primer composition (i.e. a primer coat) on at least a part of the metal surface, and a coat of a second coating composition (e.g. a top-coat) on at least a part of the primer coat; as well as an article coated with the primer composition.

Description

EPOXY-BASED PRIMER COMPOSITIONS FIELD OF THE INVENTION

The present invention relates to epoxy-based primer compositions having improved properties with respect to corrosive delamination from an underlying metallic substrate, and to a method for applying the primer composition to a metal substrate by spraying.

BACKGROUND

The problem underlying the invention is that of delamination/disbondment/damage of a paint coat at the interface between the metal surface (e.g. steel surface) and the primer coat (typically an epoxy primer). The delamination/disbondment is believed to be caused by an electrochemical process at the metal surface which reduces oxygen at the metal surface, thereby forming peroxides and/or superoxide or hydroxy radicals, which cause degradation of the polymer.

Leng et al. 1999 report a study of the delamination of polymeric coatings from steel. The authors state: "[..] This conclusion has been confirmed by recent studies on the delamination of ultrathin plasma polymers [..] in which it was proven that the metal :polymer interface is not destroyed by an alkaline attack but by a strongly oxidative attack which can only be caused by the intermediates of the oxygen reduction reaction. Therefore, it should be of interest to study whether radical-consuming substances, like ascorbic acid, vitamin E, Carotin(oids) or other radical-quenching molecules as polymer additives, are able to markedly retard the delamination rate."

JP 05140503 (MAT) relates to metal coating compositions which have cathodic disbondment- proofing properties and waterproofing adhesive properties. It is disclosed that: "It is arbitrary to add auxiliary agents, such as a catalyst for promoting a hardening reaction, an

antioxidant, and stabilizer." US 3,578,615 discloses epoxy resin powder coating having improved cathodic disbondment resistance. The compositions comprise a polyepoxide, an epoxy curing agent, a catalyst and a bonding additive (e.g. barium sulphate, strontium chromate, o-nitrophenol, phosphoric acid, and aminosilanes).

US 4,612,049 discloses corrosion-inhibiting coating compositions comprising aliphatic or cycloaliphatic carboxylic acids containing a heterocyclic radical. US 4,818,777 and US 4,894,091 disclose phenolic corrosion inhibitors for coating materials comprising phenolic derivatives of benzothiazole or mercaptobenzothiazole.

EP 385941 A2 discloses corrosion-resistant compositions comprising N-substituted pyrrole derivatives. N-(2-Hydroxyphenyl)-2,5-dimethylpyrrole is one of several examples. US 5,624,979 discloses phosphorous-modified epoxy resins.

In view of the above, there is a need for liquid epoxy-based anticorrosive primer

compositions having improved properties with respect to suppression of

delamination/disbondment, in particular for application to a metal substrate by spraying.

DESCRIPTION OF THE INVENTION The present inventors have now found that the above-mentioned problems can be overcome by the inclusion of one or more particular phenolic compounds into the anticorrosive primer composition, in particular a primer composition for application to a metal substrate by spraying.

Hence, the present invention provides a method of applying a liquid epoxy-based anti- corrosive primer composition comprising one or more free-radical scavengers in an amount of 0.5-20 % by weight, wherein the free-radical scavengers are selected from specifically defined phenols and phenol multimers, to at least at part of the surface of a metal substrate by spraying.

Without being bound to one specific theory, it is however believed that cathodic delamination of organic coatings from metallic surfaces is, at least in part, due to a chemical attack of the coating by free radicals, namely superoxide and hydroxy radicals, which are formed as an intermediate during the electrochemical reduction of oxygen on the cathodically polarized steel surface. The aforementioned degradation of the coating by free radicals can be minimized by incorporation of free-radical scavengers into the primer of the coating system. The free-radical scavengers

The selection of the free-radical scavengers appears to be of utmost importance. In one currently preferred embodiment of the invention, the free-radical scavengers are selected from (a) phenols which are substituted with one or more electron-donating groups selected from Ci-6-alkyl optionally substituted with a substituent selected from phenyl, C₁-24- alkyloxycarbonyl, Ci-24-alkylcarbonyloxy, Ci-24-alkylaminocarbonyl, Ci-24-alkylcarbonylamino, di(Ci-₆-alkyl)phosphono-oxy, Ci-₂₄-alkoxy, and Ci-₂₄-alkylthio; phenyl optionally substituted with one or more Ci-6-alkyl ; hydroxy; Ci-6-al koxy; Ci-6-al kylthio; amino; Ci-6-alkylamino; and di(Ci-6-alkyl)amino.

Preferably, the phenol is substituted with at least one group selected from optionally substituted Ci-₆-al kyl, e.g . Ci-₆-alkyl optionally substituted with phenyl . If the phenol is exclusive substituted with optionally substituted Ci-₆-alkyl, preferably at least one C₂-6-alkyl is present in the ortho position relative to the phenol functionality.

In one embodiment hereof, the phenol is substituted with one or more groups selected from hydroxy and Ci-₆-alkyl . In one variant hereof, the phenol is substituted with at least one or more groups selected from Ci-6-al kyl . In one specific variant hereof, the phenol is substituted with at least one tert-butyl group.

Furthermore, at least one substituent is preferably ortho or para, in particular ortho, relative to the phenol functionality.

Illustrative examples of interesting free-radical scavengers are the phenols selected from 2,5- di(tert-butyl)-hydroquinone, 4-tert-butyl-catechol, tert-butyl-hydroquinone, 2,6-di(tert- butyl)-4-methyl-phenol, 2,6-di(tert-butyl)-phenol, 2-tert-butyl-4,6-dimethyl-phenol, 2-tert- butyl-4-methoxy-phenol, l-(2-hydroxyphenyl)-l-phenyl -ethane, 2,6-di(tert-butyl)-4- (phosphono-oxymethyl)-phenol, 2,6-di(tert-butyl)-4-(iso-octyloxycarbonylethyl)-phenol, 4- amino-phenol, 2, 6-diphenyl-4-methyl -phenol, 2,4-bis(C₅-i₂-alkylthiomethyl)-6-methyl .

Particularly interesting examples are 2,5-di(tert-butyl)-hydroquinone, tert-butyl- hydroquinone, 4-tert-butyl-catechol, 2,6-di(tert-butyl)-phenol, 2,6-di(tert-butyl)-4-methyl- phenol, 2-tert-butyl-4-methoxy-phenol .

For the sake of cost efficiency, it is believed that the free-radical scavenger should be represented by a fairly uncomplicated structure. As a consequence hereof, the phenols to be used in the context of the present invention preferably have a phenol equivalent weight of less than 400 g/mol, preferably less than 350 g/mol .

The "phenol equivalent weight" is defined as the molar mass of the free-radical scavenger for free-radical scavengers that contain a single phenol functionality. Additional hydroxyl groups attached to the same benzene ring cannot act as radical scavengers and are therefore not considered . The phenol equivalent weight for free-radical scavengers with multiple phenol groups is defined as the molar mass of the free-radical scavenger divided by the number of phenol groups. In another currently preferred embodiment of the invention, the free-radical scavengers are selected from (b) phenol multimers of the formula Z(Y)_n, wherein n is 2, 3, 4 or 5; and wherein Z is a central scaffold having a valence of n and having covalently bonded thereto n phenol moieties, Y, each phenol moiety, Y, independently being optionally substituted with one or more electron-donating groups selected from Ci-6-alkyl optionally substituted with a substituent selected from phenyl, Ci-24-alkyloxycarbonyl, Ci-24-alkylcarbonyloxy, C₁-24- alkylaminocarbonyl, Ci-24-alkylcarbonylamino, di(Ci-₆-alkyl)phosphono-oxy, Ci-24-alkoxy, and Ci-24-alkylthio; phenyl optionally substituted with one or more Ci-₆-alkyl; hydroxy; Ci-₆- alkoxy; Ci-₆-alkylthio; amino; Ci-6-alkylamino; and di(Ci-6-alkyl)amino. In connection with the free-radical scavengers of the phenol multimer type of the formula

Z(Y)_n, the expression "optionally substituted" is intended to mean that the benzene ring may or may not carry further substituents of the specified electron-donating types in addition to the -OH group and the bond to the scaffold "Z".

If none of the phenol moieties are substituted, then the scaffold preferably is not optionally substituted methylene.

The central scaffold of the phenol multimer may be represented by a single atom or of a moiety, each having a valence, n, corresponding to that of the number of phenols covalently attached thereto.

In one embodiment, the central scaffold, Z, is selected from Ci-₆-alkylene, -0-, -S-, and -N< . In one variant hereof, the central scaffold, Z, is selected from Ci-6-alkyl-l,l-ene, such as methylene, ethyl-l,l-ene, propyl-l,l-ene, propyl-2,2-ene, butyl-l,l-ene, and 2-methyl- propyl-l,l-ene. Illustrative examples of interesting free-radical scavengers of this type are the phenol multimer selected from bis(3,5-di(tert-butyl)-4-hydroxyphenyl)methane, bis(3-tert-butyl-5- methyl-2-hydroxyphenyl)methane, bis(3-tert-butyl-5-ethyl-4-hydroxyphenyl)methane, 1,1- bis(3-tert-butyl-6-methyl-4-hydroxyphenyl)-butane, 2-methyl-l,l-bis(3,5-dimethyl-2- hydroxyphenyl)-propane, and l,l,3-tris(3-tert-butyl-6-methyl-4-hydroxyphenyl)-butane. In another variant, the central scaffold, Z, is selected from -0-, -S-, and -N< . Illustrative examples of interesting free-radical scavengers of this type are the phenol multimer selected from di(3-tert-butyl-2-hydroxyphenyl)sulfide, and tri(4- hydroxyphenyl)amine, tri(3-hydroxyphenyl)amine.

In another embodiment, the central scaffold, Z, is selected from l,3,5-trimethylene-2,4,6-tri- tert-butylbenzene (2,4,6-(CH₃)₃-benzene-l,3,5-((CH₂)-)₃), l,3,5-trimethylene-2,4,6-tri-tert- butylbenzene (2,4,6-((CH₃)₃C)₃-benzene-l,3,5-((CH₂)-)₃), and pentaerythritol tetrakis(3- propionate) (C(CH₂-0-C(=0)-CH₂-)₄)-

Other illustrative examples of free-radical scavengers of the phenol multimer type are tetra(4-hydroxy-3,5-di(tert-butyl)phenylmethylcarbonyloxymethyl)methane, tetra(4- hydroxy-3,5-di(tert-butyl)phenylethylcarbonyloxymethyl)methane, l,3,5-tris(4-hydroxy-3,5- di (tert-butyl)phenylmethyl)-2, 4, 6-trimethyl -benzene, l,2-bis(3,5-di-tert-butyl-4-hydroxy- hydrocinnamoyl) hydrazide, triethylene glycol bis(3-tert-butyl-4-hydroxy-5-methylphenyl)- propionate, 3,5-bis(l, l-dimethylethyl)-4-hydroxybenzenepropanoic acid thiodi-2,1- ethanediyl ester, 3,3'-bis(3,5-di-tert-butyl-4-hydroxyphenyl)-N,N'-hexamethylene- dipropionamide, ethylene(3,3-bis(3-(l,l-dimethylethyl)-4-hydroxyphenyl)butanoate.

As above, the phenyl multimer should preferably be represented by a fairly uncomplicated structure. As a consequence hereof, the phenol multimers to be used in the context of the present invention should preferably have a phenol equivalent weight of less than 400 g/mol, preferably as less than 350 g/mol . Preferably, the one or more free-radical scavengers are present in a total amount of 0.5-20 % by weight, in particular in a total amount of 1.0-15 % by weight. In some embodiments, the one or more free-radical scavengers are present in a total amount of 2.0-20 % by weight, e.g . 2.5-18 % by weight, or 3.0-16 % by weight, or 3.5-14 % by weight, or 4.0-18 % by weight, or 4.5-16 % by weight, or 5.0-14 % by weight, or 1.5-10 % by weight, or 2.0-8.0 % by weight, or 2.5-7.0 % by weight, or 3.0-6.0 % by weight.

It is believed that the one or more free-radical scavengers should be rather sparsely soluble in water. Hence, preferably, each of the one or more free-radical scavengers have a solubility in water of less than 0.2 g/mL at 25 °C, in particular less than 0.1 g/mL at 25 °C.

Moreover, the free-radical scavengers to be used within the present invention are preferably solid at a temperature of 25 °C.

In the present context, the terms "Ci-i₂-alkyl" and "Ci-₆-alkyl" are intended to mean a linear, cyclic or branched hydrocarbon group having 1 to 12 carbon atoms and 1 to 6 carbon atoms, respectively, such as methyl, ethyl, propyl, /so-propyl, pentyl, cyclopentyl, hexyl, cyclohexyl, octyl, dodecanyl, etc. The term "alkoxy" means "alkyl-oxy" ("alkyl-O"), e.g. Ci-24-alkoxy means Ci-24-alkyl-oxy. Similarly, the term "alkylthio" means "alkyl-S-".

The liquid epoxy-based anti-corrosive primer composition

The liquid epoxy-based anti -corrosive primer composition may - beside the above-specified free-radical scavengers - be of any conventional type.

The term "epoxy-based primer system" should be construed as the combination of the one or more epoxy resins, one or more curing agents, any reactive epoxy diluents and any reactive acrylic modifiers. The epoxy-based primer system is one of the most important constituents of the paint composition, in particular with respect to the anticorrosive properties.

The epoxy-based primer system comprises one or more epoxy resins selected from aromatic or non-aromatic epoxy resins (e.g. hydrogenated epoxy resins), containing more than one glycidyl group per molecule, which is placed internally, terminally, or on a cyclic structure, together with one or more suitable curing agents to act as cross-linking agents. Combinations with reactive diluents from the classes mono functional glycidyl ethers or esters of aliphatic, cycloaliphatic or aromatic compounds can be included in order to reduce viscosity and for improved application and physical properties. The primer system can also include reactive acrylic modifiers, such as acrylate monomers and oligomers comprising at least two alpha, beta unsaturated carbonyl groups, reacting with the one or more curing agents via a Michael- type addition reaction.

Suitable epoxy-based primer systems are believed to include epoxy and modified epoxy resins selected from bisphenol A, bisphenol F, Novolac epoxies, non-aromatic epoxies, cycloaliphatic epoxies, glycidyl esters and epoxy functional acrylics or any combinations hereof. Examples of suitable commercially available epoxy resins are : Epikote 828, ex.

Resolution Performance Products (The Netherlands), bisphenol A type; Araldite GY 250, ex. Huntsman Advanced Materials (Switzerland), bisphenol A type; Epikote 1004, ex. Resolution Performance Products (Germany), bisphenol A type; DER 664-20, ex. Dow Chemicals (Germany), bisphenol A type; Epikote 1001 X 75, ex. Resolution Performance Products (The Netherlands), bisphenol A type; Araldite GZ 7071X75BD, ex. Huntsman Advanced Materials (Germany), bisphenol A type; DER 352, ex. Dow Chemicals (Germany), mixture of bisphenol A and bisphenol F; Epikote 232, ex. Resolution Performance Products (The Netherlands), mixture of bisphenol A and bisphenol F; Epikote 862, ex. Resolution Performance Products (The Netherlands), bisphenol F type; DEN 438-X 80, ex. Dow Chemical Company (USA), epoxy novolac; and Epikote 154, ex. Resolution Performance Products (The Netherlands), epoxy novolac.

The epoxy-based primer system comprises one or more curing agents selected from compounds or polymers comprising at least two reactive hydrogen atoms linked to nitrogen.

Suitable curing agents are believed to include amines or amino functional polymers selected from aliphatic amines and polyamines (e.g. cycloaliphatic amines and polyamines), amidoamines, polyamidoamines, polyoxyalkylene amines (e.g. polyoxyalkylene diamines), aminated polyalkoxyethers (e.g. those sold commercially as "Jeffamines"), alkylene amines (e.g. alkylene diamines), aralkylamines, aromatic amines, Mannich bases (e.g. those sold commercially as "phenalkamines"), amino functional silicones or silanes, and including epoxy adducts and derivatives thereof.

Examples of suitable commercially available curing agents are: Cardolite NC-541, ex.

Cardanol Chemicals (USA), Mannich base; Cardolite Lite 2001, ex. Cardanol Chemicals (USA), Mannich base; Epikure 3140 Curing Agent, ex. Resolution Performance Products (USA), polyamidoamine; SIQ Amin 2030, ex. SIQ Kunstharze GmbH (Germany), polyamidoamine;

Epikure 3115X-70 Curing Agent, ex. Resolution Performance Products (USA),

polyamidoamine; SIQ Amin 2015, ex. SIQ Kunstharze GmbH (Germany), polyamidoamine;

Polypox VH 40309/12, ex. Dow Chemicals (Germany), polyoxyalkylene amine; CeTePox 1490 H, ex. CTP Chemicals and Technologies for Polymers (Germany), polyoxyalkylene amine;

Epoxy hardener MXDA, ex. Mitsubishi Gas Chemical Company Inc (USA), aralkyl amine;

Diethylaminopropylamine, ex. BASF (Germany), aliphatic amine; Gaskamine 240, ex.

Mitsubishi Gas Chemical Company Inc (USA), aralkyl amine; Cardolite Lite 2002, ex.

Cardanol Chemicals (USA), Mannich base; Aradur 42 BD, ex. Huntsman Advanced Materials (Germany), cycloaliphatic amine; Isophorondiamin, ex. BASF (Germany), cycloaliphatic amine; Epikure 3090 Curing Agent, ex. Resolution Performance Products (USA),

polyamidoamine adduct with epoxy; Crayamid 147, ex. Cray Valley (Italy), amidoamine;

Aradur 943 CH, ex. Huntsman Advanced Materials (Switzerland), alkylene amine adduct with epoxy; and Aradur 863 XW 80 CH, ex. Huntsman Advanced Materials (Switzerland), aromatic amine adduct with epoxy.

Preferred epoxy-based primer systems comprises a) one or more epoxy resins selected from bisphenol A, bisphenol F and Novolac; and b) one or more curing agents selected from Mannich Bases, polyamidoamines, polyoxyalkylene amines, alkylene amines, aralkylamines, polyamines, and adducts and derivatives thereof. Especially preferred epoxy-based primer systems comprise one or more bisphenol A epoxy resins and one or more curing agents selected from Mannich Bases, polyamidoamines and adducts and derivatives thereof.

Preferably, the epoxy resin has an epoxy equivalent weight of 100-2000, such as 100-1500, e.g. 150-1000, such as 150-700.

Especially preferred epoxy-based primer systems comprise one or more bisphenol A epoxy resins having an epoxy equivalent weight of 150-700 and one or more polyamidoamine or adducts and derivatives thereof.

The epoxy-based primer systems are ambient curing primer systems, i.e. the primer composition is curable at a temperature in the range from 0 °C to 50 °C, such as from 2 °C to 40 °C, e.g. from 5 °C to 30 °C.

In the paint composition, the total amount of epoxy-based primer system is in the range of 15-80%, such as 35-80%, e.g. 40-75% by solids volume of the paint.

Without being bound to any particular theory, it is believed that the selection of the ratio between the hydrogen equivalents of the one or more curing agents and the epoxy equivalents of the one or more epoxy resins plays a certain role for the performance of the coating composition.

When used herein, the term "hydrogen equivalents" is intended to cover only reactive hydrogen atoms linked to nitrogen. The number of "hydrogen equivalents" in relation to the one or more curing agents is the sum of the contribution from each of the one or more curing agents. The contribution from each of the one or more curing agents to the hydrogen equivalents is defined as grams of the curing agent divided by the hydrogen equivalent weight of the curing agent, where the hydrogen equivalent weight of the curing agent is determined as: grams of the curing agent equivalent to 1 mole of active hydrogen. For adducts with epoxy resins the contribution of the reactants before adductation is used for the determination of the number of "hydrogen equivalents" in the epoxy-based primer system.

The number of "epoxy equivalents" in relation to the one or more epoxy resins is the sum of the contribution from each of the one or more epoxy resins. The contribution from each of the one or more epoxy resins to the epoxy equivalents is defined as grams of the epoxy resin divided by the epoxy equivalent weight of the epoxy resin, where the epoxy equivalent weight of the epoxy resin is determined as: grams of the epoxy resin equivalent to 1 mole of epoxy groups. For adducts with epoxy resins, the contribution of the reactants before adductation is used for the determination of the number of "epoxy equivalents" in the epoxy- based primer system. It should be understood that if the epoxy-based primer system contains reactive acrylic modifiers then the number of "epoxy equivalents" is to be increased accordingly. E.g. if the epoxy-based primer system contains an acrylate oligomer comprising alpha, beta

unsaturated carbonyl groups then the number of "alpha, beta unsaturated carbonyl group equivalents" are to be added to the epoxy equivalents of the one or more epoxy resins for the purpose of establishing the ratio between the hydrogen equivalents of the one or more curing agents and the epoxy equivalents of the one or more epoxy resins.

Preferably, the ratio between the hydrogen equivalents of the one or more curing agents and the epoxy equivalents of the one or more epoxy resins is in the range of 20 : 100 to 120 : 100.

Especially preferred epoxy-based primer systems have a ratio between the hydrogen equivalents of the one or more curing agents and the epoxy equivalents of the one or more epoxy resins in the range of 60 : 100 to 120 : 100, such as 70 : 100 to 110 : 100.

For the purpose of facilitating easy application of the primer composition (e.g. by spray, brush or roller application techniques), the primer composition typically has a viscosity in the range of 25-25,000 mPa-s, such as in the range of 150-15,000 mPa-s, in particular in the range of 200-1000 mPa-s. In contrast hereto, coating compositions adapted for powder coating are solids and have no measurable viscosity. In the present application with claims, viscosity is measured at 25 °C in accordance with ISO 2555 : 1989.

Adhesion promotors

It has been found that the primer composition advantageously may further comprise one or more adhesion promoters, especially of the silane type.

Adhesion promoters of the silane type are organosilicon compounds that have two different functional groups, including one that reacts with organic materials and one that reacts with inorganic materials. This unique characteristic enables them to bond organic materials (coatings) to inorganic materials (substrate) . Silanes could have a wide variety of organic functional groups and chemical reactivities. The organic functional groups can include, epoxy, amino, ketimino, vinyl, methacryloxy, acryloxy, mercapto, polysulfido, isoyanato, styryl, as well as other organic groups. Silanes can boost mechanical strength of compound materials, improve moisture resistance and adhesion.

Adhesion promoters may be selected from organofuntional silanes of the general formula : Y-R¹-Si(R²)n(R³)3 n_/ wherein n is 0, 1 or 2, in particular 0 or 1; R¹ is selected from Ci-s- alkylene (e.g. methyl, ethyl, hexyl, octyl, etc.), Ci-₄-alkylene-0-C₂-4-alkyl; arylene (e.g.

phenyl) and arylene-Ci-₄-alkylene (e.g. benzyl), etc. ; R² is selected from Ci-₈-alkyl (e.g.

methyl, ethyl, hexyl, octyl, etc.), Ci-₄-alkyl-0-C₂-4-alkyl; aryl (e.g. phenyl) and aryl-Ci-₄-alkyl (e.g. benzyl), etc. ; R³ is a hydrolysable group, e.g. methoxy, ethoxy, 2-methoxy-ethoxy, etc. ; and Y is a reactive substituent, e.g. epoxy, amino, ketimino, vinyl, methacryloxy, acryloxy, mercapt, polysulfido, isoyanato, styryl, etc.

Illustrative examples of such adhesion promoters are: a) Epoxysilanes, e.g. of the formula A-Si(R²)_n(R³)(3-_n)_/ wherein A is an epoxide-substituted monovalent hydrocarbon radical having 2 to 12 carbon atoms; and each of n, R² and R³ are as above. The group A in the epoxysilane is preferably a glycidoxy-substituted alkyl group, for example 3-glycidoxypropyl. The epoxysilane can for example be 3-glycidoxypropyltri- methoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropyldiethoxymethoxysilane, 2- glycidoxypropyltrimethoxysilane, 3-(3,4-epoxycyclohexyl)propyltrimethoxysilane, 2-(3,4- epoxy-4-methylcyclohexyl)-ethyltrimethoxysilane, 5,6-epoxy-hexyltriethoxysilane.

Commercially available epoxysilanes are 5,6-epoxy-hexyl triethoxysilane (ABCR GmbH & Co. KG, Germany); 3-glycidoxypropyl methyldiethoxysilane (ABCR GmbH & Co. KG, Germany), γ-glycidoxypropyyltrimethoxysilane (Dynasylan, Glymo, Sivento Chemie GmbH, Germany). b) Methacrylatesilanes: 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyl - trimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyl - triethoxysilane. c) Mercaptosilanes: 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxy- silane. d) Aminosilanes, e.g. of the formula R⁴NH-R¹-Si(R²)_n(R³)3 n_/ wherein each of n, R¹, R² and R³ are as defined above, and R⁴ is selected from hydrogen and - (CH₂)2-4-NH₂. Illustrative examples of aminosilanes are (CH₃0)3Si (CH2)3N H (CH2)2N H₂ ; (CHaCHzOCHzCHzOJsS CHzk- NH₂; (C₂H50)3Si (CH2)3N H₂; (CHaOCHzCHzOJsS CHzJsN Hz ; (CzHsOJsS CHzJsC CHzJsN Hz ;

(CzHsOJsSiCHzC CHzJzN Hz ; (CzHsOJsS CHzJsC CHzJzN Hz ; and (C2H₅0)2CH₃Si(CH2)3N H2. Illustrative examples of commercially available aminosilanes are Dynasilan AMEO (3-aminopropyltriethoxysilane) ex Evonik Degussa; KBM603 (Ν-β- aminoethyl-Y-aminopropyltrimethoxysilane) ex Shin Etsu; etc. e) Dipodal silanes and bis-sulfur silanes, e.g. of the formula R₃Si-(CH2)n-R-(CH₂)n-SiR₃ and R₃Si-(CH2)n-R-S-S-R-(CH₂)n-SiR₃. Illustrative examples of such silanes are bis[3-(triethoxy- silyl)propyl]disulfide, bis[3-(triethoxysilyl)propyl]tetrasulfide, bis(3-trimethoxysilylpropyl)- amine; N,N'-bis[3-(trimethoxysilyl)propyl]ethylene-diamine; and bis(triethoxysilyl)ethylene. Illustrative examples of commercial dipodal and bis-sulfur silanes are SIB 1825.0 and SIB1824.6 from Gelest, Inc.

When included, the adhesion promoter is typically present in an amount of 0-10 % by weight, in particular in an amount of 0.1-8% by weight.

Other constituents

The paint composition may comprise plasticizers. Examples of plasticizers are hydrocarbon resins, phthalates and benzyl alcohol. In one preferred embodiment, the paint composition comprises a hydrocarbon resin as plasticizer. In the paint composition, the total amount of plasticizers (e.g. hydrocarbon resins) may be in the range of 0-30%, such as 0-25% by solids volume of the paint, preferably 1-25%, such as 1-20% by solids volume of the paint.

The paint composition may comprise other paint constituents as will be apparent for the person skilled in the art. Examples of such paint constituents are pigments, fillers, additives (e.g. epoxy accelerators, surfactants, hydroxy-functional modifying resins, wetting agents and dispersants, de-foaming agents, catalysts, stabilizers, corrosion inhibitors, coalescing agents, thixothropic agents (such as polyamide waxes), anti-settling agents and dyes).

In the paint composition, the total amount of pigments and fillers may be in the range of 0- 50%, such as 5-50% by solids volume of the paint, preferably 10-45%, such as 10-40% by solids volume of the paint.

It is envisaged that certain pigments and fillers have a beneficial effect on the anticorrosive properties. Examples are aluminium pigments, zinc phosphate and mica. In one preferred embodiment, the paint composition comprises 0-10% by solids volume of the paint of aluminium pigments, preferably 1-7%, such as 2-6% by solids volume of the paint. In an alternative embodiment, the composition comprises at the most 10% by dry weight of the paint of aluminium pigments. In the paint composition, the total amount of additives may be in the range of 0-10%, such as 0.1-8% by solids volume of the paint.

The paint composition may further comprise epoxy accelerators.

The paint composition typically comprises a solvent or solvents. Examples of solvents are alcohols, such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol and benzyl alcohol; aliphatic, cycloaliphatic and aromatic hydrocarbons, such as white spirit,

cyclohexane, toluene, xylene and naphtha solvent; ketones, such as methyl ethyl ketone, acetone, methyl isobutyl ketone, methyl isoamyl ketone, diacetone alcohol and cyclo- hexanone; ether alcohols, such as 2-butoxyethanol, propylene glycol monomethyl ether and butyl diglycol; esters, such as methoxypropyl acetate, n-butyl acetate and 2-ethoxyethyl acetate; and mixtures thereof.

Depending on the application technique, it is desirable that the paint comprises solvent(s) so that the solids volume ratio (SVR - ratio between the volume of solid constituents to the total volume) is in the range of 30-100%, preferably 50-100%, in particular 55-100% e.g. 60- 100%.

SVR is determined according to ISO 3233 or ASTM D 2697 with the modification that drying is carried out at 20°C and 60% relative humidity for 7 days instead of drying at higher temperatures.

It has previously been common to include coal-tar in epoxy paint compositions. However due to a suspected carcinogenic effect it is preferred that epoxy paint compositions do not contain coal-tar. The presence of coal-tar furthermore makes it difficult to make light shades. Light shades are preferred in ballast-tanks to facilitate inspection for possible damages to the paint film. Thus, in a preferred embodiment the coating composition comprises 0% by solids volume of the paint of coal-tar. Preparation of the primer composition

The primer composition may be prepared by any suitable technique that is commonly used within the field of paint production. Thus, the various constituents may be mixed together using a high speed disperser, a ball mill, a pearl mill, a three-roll mill etc. The paints according to the invention may be filtrated using bag filters, patron filters, wire gap filters, wedge wire filters, metal edge filters, EGLM turnoclean filters (ex. Cuno), DELTA strain filters (ex. Cuno), and Jenag Strainer filters (ex. Jenag), or by vibration filtration. Hence, the primer composition to be used in the method of the invention is prepared by mixing two or more components, e.g. two pre-mixtures, one component, e.g. a pre-mixture, comprising the one or more epoxy resins and one component, e.g. a pre-mixture, comprising the one or more curing agents. It should be understood that when reference is made to the primer composition, it is the mixed primer composition ready to be applied. Furthermore, all amounts stated as % by solids volume of the paint should be understood as % by solids volume of the cured composition after application and drying.

An example of a suitable preparation method is described in the Examples.

Application of the coating composition

The primer composition of the invention is typically applied to at least a part of the surface of a substrate.

The term "applying" is used in its normal meaning within the paint industry. Thus, "applying" is conducted by means of any conventional means, e.g. by brush, by roller, by spraying, by dipping, etc. The commercially most interesting way of "applying" the coating composition is by spraying, however, often in combination with brush application on touch-up areas and/or on edges, corners etc. in the paint business named "stripe coating".

The primer composition is adapted for application by spraying, i.e. the viscosity thereof allows for application using conventional spraying equipment.

Spraying is effected by means of conventional spraying equipment known to the person skilled in the art. The primer composition is typically applied in a dry film thickness of 100- 600 μηη per coat, such as 150-450 μηη, e.g. 200-400 μηη, but could also be as low as 30 μηη and as thick as 3000 μηη per coat. In shop application the so called "shop primers" are typically applied in a thickness of 15-20 μηη.

The term "at least a part of the surface of a substrate" refers to the fact that the primer composition may be applied to any fraction of the surface. For many applications, the primer composition is at least applied to the part of the substrate (e.g. a vessel) where the surface (e.g. the ship's hull), possibly after application of further coating layers, may come in contact with water, e.g. sea-water or another corrosive environment.

The term "substrate" is intended to mean a solid material onto which the coating composition is applied. The substrate comprises a metal such as steel (i.e. carbon steel and stainless steel alloys), electro galvanized steel, hot dip galvanized steel, thermally spray metallized steel, copper and copper alloys, aluminium and aluminium alloys. In the most interesting embodiments, the substrate is a metal substrate, in particular a steel substrate. In some embodiments, the substrate is at least a part of the outermost surface of a structure exposed to a corrosive environment. The term "surface" is used in its normal sense, and refers to the exterior boundary of an object. Particular examples of such surfaces are the surface of marine structures, such as marine vessels (including but not limited to boats, yachts, motorboats, motor launches, ocean liners, vessels for floating production storages and offload (FPSOs), workboats, tugboats, tankers, container ships and other cargo ships, submarines, naval vessels of all types and barges), tanks, such as ballast water tanks, on-shore or off-shore storage tanks for water or water/oil mixtures), pipelines, such as oil and/or gas transporting pipelines immersed in water or soil), off-shore installations and structures, such as oil and gas producing installations, platforms, under-water oil well structures, off-shore wind-turbines, water power installations and structures, other constructions and objects of all types, such as piers, pilings, bridge substructures and other aquatic culture installations, and buoys, etc.

A coating system

The present invention provides a single paint coat on a metal surface, comprising coat of an epoxy-based primer composition. But it also provides a coating system on a metal surface, comprising coat of an epoxy-based primer composition (i.e. a primer coat) on at least a part of the metal surface, and one of multiple coats of another coating composition (e.g.

combinations of one or multiple mid coats and a top-coat) on at least a part of the primer coat, wherein the primer composition is as defined herein. The coating system is typically established by application of the primer composition as described above.

In one embodiment, the coat of the second coating composition represents a non-transparent coat.

A coated article

The present invention also relates to an article coated with a primer composition as defined hereinabove. Such article may without limitations encompass marine vessels, bridges, containers, tanks, pipes, offshore installations, etc. The primer composition is typically applied in accordance with the method described above.

Marine vessels

The present invention also provides a marine structure comprising on at least a part of the outer surface thereof a primer coating prepared from a coating composition as defined hereinabove. The primer composition is typically applied in two coats in a dry film thickness of 100-200 μηη per coat, such as 125-175 μηη, e.g. 150 μηη.

Tanks

The coating system of the internal tank system may consist of 2-3 coats of an anticorrosive layer of 100-200 μηη per coat. The primer composition of the external system may consist of 1-3 coats of an anticorrosive layer of 200-600 μηη per coat or even 1 coat in a dry film thickness of 1000-3000 μιτι.

Pipes

The primer composition of the internal system may consist of 2-3 coats of an anticorrosive layer of 100-200 μηη per coat. The primer composition of the external system may consist of 1-3 coats of an anticorrosive layer of 200-600 μηη per coat or even 1 coat in a dry film thickness of 1000-3000 μιτι.

Offshore installations

In the splash zone the coating system may consist of 2-3 coats of an anticorrosive layer of 100-500 μηη per coat, or the primer composition may consist of 1-3 coats of an anticorrosive layer of 200-600 μηη per coat or even 1 coat in a dry film thickness of 1000-3000 μηη. Below the splash zone the coating system may consist of 2-3 coats of an anticorrosive layer of 100- 200 μηη per coat.

Other constructions and objects

For immersed systems a typically specification may consist of 2-3 coats of an anticorrosive layer of 100-200 μηη per coat.

General Remarks

Any combination of two or more of the embodiments described herein is to be construed as falling within the scope of the present invention. Although the present description and claims occasionally refer to a primer, a scavenger, etc., it should be understood that the coating compositions defined herein may comprise one, two or more types of the individual constituents. In such embodiments, the total amount of the respective constituent should correspond to the amount defined above for the individual constituent. The "(s)" in the expressions: scavenger(s), polysiloxane(s), etc. indicates that one, two or more types of the individual constituents may be present. On the other hand, when the expression "one" is used, only one (1) of the respective constituent is present. EXAMPLES

Test Methods

Cathodic Protection Test

The Cathodic Protection Test is carried out in accordance with ISO 15711: Determination of resistance to cathodic disbonding of coatings exposed to sea water (method A, Impressed current). This test method simulates the conditions experienced in real life by cathodically- protected structures immersed in sea water, e.g., ship hulls, ballast tanks or offshore structures. Before testing, the coating is artificially damaged in the form of a bare spot (holiday) of 6 mm in examples series B, and 11 mm in examples series A, in diameter located approximately in the middle of the panel. The panels are exposed in a test tank containing circulating artificial sea water at 17°C. The electrical stress is produced by connecting the panels to a cathodic protection circuit. The panels are cathodically polarized at -1050 mV_SCE (saturated calomel electrode). Inspections are carried out regularly over the testing time until completion of the exposure. At each inspection, blisters are reported according to ISO 4628-2 by using a template which divides the area of the panel into four zones, corresponding to four rings of different radial length distributed from the perimeter of the holiday outwards (Figure 1 - illustrating "Zones for blistering evaluation"). The panels are exposed for 9 weeks in example series B and 8 weeks in examples series A. The evaluation of the panels is done immediately at completion of the exposure by making two radial cuts through the coating at 45° to each other approximately, intersecting at the centre of the holiday. The disbonding is determined by picking off the paint with a sharp knife or scalpel from the edge of the holiday outwards. The maximum radial length of disbonding is measured and reported in mm from the edge of the holiday.

Cathodic Protection Test in KCI

This test method is a slight variation on ISO 15711 in that the electrolyte is a 0.4 M solution of KCI instead of the artificial sea water described in the standard. The test is conducted at standard laboratory temperature (23°C approximately). Blister Box Test

The Blister Box Test is carried out according to ISO 6270-1 : Determination of resistance to humidity (Part 1, continuous condensation). This method is performed in order to evaluate the water resistance of a coating system by exposing it to controlled condensation conditions. The panel surface with the coating system is exposed to saturated water vapour at 40°C and at an angle of 15° to 60° with the horizontal. The reverse side of the panel is exposed to room temperature (23°C approximately). The panels are exposed for 9 weeks. At completion of the exposure, blistering and rust are evaluated according to ISO 4628-2 and ISO 4628-3, respectively.

Salt Spray Test

The Salt Spray Test is conducted according to ISO 9227: Corrosion tests in artificial atmospheres - Salt spray tests. This method is performed in order to evaluate the corrosion resistance of a coating system by reproducing the corrosion that occurs in atmospheres containing salt spray or splash. Before testing, the coating is artificially damaged by making a 2-mm-wide scribe line of 50 mm in length. The scribe line is 20 mm above the bottom of the panels, with a spacing of 10 mm on either side. The panels are exposed for 9 weeks in examples series B, and 26 weeks in examples series A, to constant salt spray at 35°C in a spray cabinet. The salt fog is generated by using a NaCI solution of 50 g/l. At completion of the exposure, blistering and rust are evaluated on both panel and around the scribe (in mm from centre), according to ISO 4628-2 and ISO 4628-3, respectively. At the scribe, the paint is picked off and the degree of under-film corrosion and delamination is determined according to ISO 4628-8.

Sea Water Immersion Test

The sea water immersion test is carried out according to ISO 2812-2: Determination of resistance to liquids (Part 2, Water immersion method). The panels are immersed in a test tank with circulating, aerated artificial sea water in accordance with ISO 15711, at 40±1°C. Before starting the exposure, the coating is artificially damaged by making a 2-mm-wide scribe line of 50 mm in length. The scribe line is 20 mm above the bottom of the panels, with a spacing of 10 mm on either side. The panels are exposed for 9 weeks in total. At completion of the exposure, blistering and rust are evaluated on both panel and around the scribe (in mm from centre), according to ISO 4628-2 and ISO 4628-3, respectively. At the scribe, the paint is picked off and the degree of under-film corrosion and delamination is determined according to ISO 4628-8.

KCI Immersion Test

This test method is a slight variation of the sea water immersion test as the electrolyte is a 0.5 M solution of KCI instead of the artificial sea water described in the standard. Before testing, the coating is artificially damaged in the form of a bare spot (holiday) of 11 mm in diameter located approximately in the middle of the panel. The panels are exposed in a test tank containing circulating artificial sea water at 17°C for 8 weeks. At completion of the exposure the paint is picked off and the degree of under-film corrosion and delamination is determined according to ISO 4628-8.

Example series A

Coating formulation

The epoxy resins used in examples series A were diglycidyl ethers of bisphenol A, either a low molecular weight epoxy resin (Epikote 828) or a medium molecular weight epoxy resin

(Epikote 1001), both supplied by Hexion. The epoxide equivalents were respectively 192 and 475 g equiv^"1. The curing agent was a polyamide adduct prepared by combining Epikote 828 with Epicure 3140. The free-radical scavengers tested were Irganox 1010 and Irganox 1135, both supplied by Ciba and DTBHQ (2,5-di-tert-butylhydroquinone) supplied by Eastman Chemicals. Irganox 1010, Irganox 1135 and DTBHQ are all sterically hindered phenols. Irgafos 168 supplied by Ciba is a triarylphosphite, i.e. a peroxide decomposer.

Coatings with compositions as shown in Table 1 were produced in batches of about 1200 mL The binder and pigment phase were mixed in a metal container and dispersed for 10-15 minutes on a high speed dissolver before addition of additional solvents and additives. All the free-radical scavengers and Irgafos 168 are practically insoluble in water (<0.01 g per 100 mL) and very soluble in organic solvents such as toluene and/or n-butanol. The free-radical scavengers and Irgafos 168 were dissolved in n-butanol prior to incorporation to ensure that they were properly mixed with the epoxy resin. The maximum particle size was measured using a grindometer and found to be 15-25 μηη for all coatings. Dispersion continued until the liquid coating reached a temperature of 70 °C. Thereafter, the kinematic viscosity of the coatings was adjusted to approximately 9-10^"3 mV¹ with a mixture of solvents. The reduced pigment volume concentration (ratio between the pigment volume concentration and the critical pigment volume concentration) was 0.6, which is a typical value for barrier coatings. Compound Content of free-radical scavengers [vol%]

0.0 (ref.) 0.5 1.0 5.0 25

Epoxy resin 33.4 33.2 33.1 33.1 23.9

Plasticizer 14.3 14.2 14.0 10.1 1.2

Leveling agent 1.2 1.2 1.2 1.2 1.2

Dispersing agent 0.5 0.5 0.5 0.5 0.5

Wetting agent 0.4 0.4 0.4 0.4 0.4

Rheological agent 1.9 1.9 1.9 1.9 1.9

Titanium dioxide 2.6 2.6 2.6 2.6 2.6

Magnesium silicate 28.6 28.5 28.5 28.5 28.5

Polyamide adduct 16.5 16.4 16.3 16.3 14.3

Epoxy accelerator 0.6 0.6 0.6 0.6 0.5

Table 1 : Composition (percentage by solids volume) of the tested coatings.

The free-radical scavengers were all tested at various levels in the range 0.5 % to 25 % per solids volume. The reduced pigment volume concentration was maintained at 0.6 for coatings with the amount of free-radical scavengers given as vol% by reducing the amount of plasticizer. When the amount of free-radical scavenger is given as percentage by weight, the free-radical scavengers were added to the reference coating. This means that the amount of filler versus the amount of binder in the coatings with the amount of free-radical scavenger given as wt% is gradually reduced as the amount of free-radical scavenger is increased.

The combined effects of free-radical scavengers and peroxide decomposers were tested by combining Irgafos 168 with Irganox 1010 or Irganox 1135 (in identical concentrations) at the same levels as the single free-radical scavengers.

The chemical structures of the applied free-radical scavengers are given below.

Irganox 1010 Irganox 1135 DTBHQ Preparation of specimens

Substrates prepared from cold rolled low alloy carbon steel (maximum 0.18 wt % carbon) were degreased with Hempel Light Clean, an alkaline detergent, and thoroughly rinsed with distilled water. Thereafter the substrates were abrasive blast cleaned with steel grit (type G9) to Sa 2¹/2 and medium roughness (R_y = 57 μηη). The coating was applied by airless spray at a pressure of 100 bars. The desired coating thickness was obtained by application of two layers of coating with similar thickness. The first coat was allowed to cure for 1 day at ambient temperature before the second coat was applied. A conventional circular sticker with a diameter of 11 mm was attached to the steel surface prior to application of the coating for substrates intended for immersion in sea water. Removal of the sticker after curing of the coating introduced a well-defined "defect". The upper right corner of specimens intended for cathodic polarization was also covered with a sticker to ensure that the cord, which was bolted to the steel surface, had electrical contact with the steel. A 2 mm wide and 5 mm long scribe was introduced by a pneumatic tool into coated specimens that were exposed to salt spray testing. A magnetic gauge instrument, Elcometer Model 355 Top, was used to measure the dry film thickness of the coatings, which was 301 μηη ± 25μηη, representative of industrial coating thicknesses. The final curing time of the coating specimens was at least 42 days at ambient temperature and three replicates were tested in all cases.

Exposure and evaluation

Immersed specimens were exposed to an aerated solution of 0.5 M sodium chloride or 0.5 M potassium chloride at either free corrosion potential or under cathodic polarization at -1050 mV (SCE). Specimens under cathodic polarization were all individually connected to the same potentiostat. The potentiostat was controlled by a computer, which continuously monitored and adjusted the cathodic potential of all specimens. A thermostat was used to maintain a constant temperature of 18 °C.

Under-film corrosion was tested in a salt spray chamber in accordance with ISO 9227. The extent of cathodic delamination or under-film corrosion was evaluated destructively by manual removal of the degraded coating with a knife. Subsequently, digital imaging software from Media Cybernetics was applied for measuring the extent of cathodic delamination or under-film corrosion.

Buffer capacity

The ability of DTBHQ and Irganox 1135 (dissolved in methanol) to neutralize potassium hydroxide was evaluated by titration with 0.1 M potassium hydroxide in methanol from Merck. Methanol has weak acid properties, therefore the amount of 0.1 M potassium hydroxide in methanol required to neutralize 100 g methanol was subtracted from the amount of 0.1 M sodium hydroxide in methanol required to neutralize 20 g free-radical scavenger in 100 g methanol.

The acid number was calculated as the amount of sodium hydroxide in milligrams required for neutralizing the phenol groups in one gram of free-radical scavenger. Results and discussion

The effect of free-radical scavengers (Irganox 1010, Irganox 1135 and DTBHQ), a peroxide decomposer (Irgafos 168) and combinations of free-radical scavengers and the peroxide decomposer on the extent of cathodic delamination of epoxy coatings based on low and medium molecular weight epoxy resin was tested at various concentrations. The results for the coatings based on a low molecular weight epoxy resin are shown in Table 2 and Table 3, at free corrosion potential and under cathodic polarization at -1050 mV (SCE), respectively. Table 4 and Table 5 show the results for the coatings based on a medium molecular weight epoxy resin, also at free corrosion potential and under cathodic polarization at -1050 mV (SCE), respectively.

Table 2: Effect of free-radical scavenger content on the extent of delamination of coating based on Epikote 1001 under cathodic polarization at -1050 mV (SCE). after 6 weeks of exposure to 0.5 M sodium chloride at 18 °C.

Free-radical Extent of delamination for various types of free-radical scavengers [mm] scavenger

[Vol %] Irgafos 168 Irganox Irganox DTBHQ Irgafos Irgafos

(reference; 1010 1135 168 + 168 + peroxide Irganox Irganox decomposer) 1010 1135

0.0 (ref.) 6.5 6.5 6.5 6.5 6.5 6.5

0.5 6.6 6.1 6.0 5.9 6.2 6.1

1.0 6.0 5.5 5.0 5.1 5.7 5.3 5.0 5.5 5.2 5.6 3.8 5.4 4.8

25.0 6.9 6.5 6.2 4.6 6.2 6.1

Table 3 : Effect of free-radical scavenger content on the extent of cathodic delamination of coatings based on Epikote 1001 at free corrosion potential after 6 weeks of exposure to 0.5 M sodium chloride at 18 °C.

Table 4: Effect of free-radical scavenger content on the extent of cathodic delamination of coatings based on Epikote 828 under cathodic polarization at -1050 mV (SCE) after 6 weeks of exposure to 0.5 M sodium chloride at 18 °C.

Table 5: Effect of free-radical scavenger content on the extent of cathodic delamination of coatings based on Epikote 828 at free corrosion potential after 6 weeks of exposure to 0.5 M sodium chloride at 18 °C. The free-radical scavengers are all capable of reducing the extent of cathodic delamination of coatings based on both low and medium molecular weight epoxy resins. This means that the high concentration of tertiary amine groups in cured coatings based on low molecular weight epoxy resins does not affect the ability of the free-radical scavengers to reduce the extent of cathodic delamination. Furthermore, the extent of cathodic delamination is reduced equally at free corrosion potential and under cathodic polarization, which indicates that free radicals are also important for cathodic delamination at free corrosion potential. The extent of cathodic delamination is reduced when the concentration of radical scavengers is increased from 0.5 vol% to 5.0 vol%, which shows that the concentration of the radical scavengers has a significant impact on cathodic delamination. At 25 vol%, the extent of cathodic delamination is increased significantly, probably because the high amount of free-radical scavengers increases the permeability of the coating towards corrosive species.

The results show that there is a limited effect of peroxide decomposer (Irgafos 168) on the extent of cathodic delamination of coatings based on low and medium molecular epoxy resins at free corrosion potential and under cathodic polarization. This indicates that the formation of peroxides do not contribute significantly to the rate of cathodic delamination of epoxy coatings.

The superior performance of DBTHQ compared with Irganox 1010 and Irganox 1135 can be partly explained in terms of molecular weight per active group. Sterically hindered phenols interact with free radicals through the phenol group. Although other factors such as steric hindrance can be important, the efficiency of sterically hindered phenols is therefore related to the equivalent weight per phenol group. Table 6 shows that the phenol equivalent weights for Irganox 1010 and Irganox 1135 are respectively 33 % and 75% greater than the molecular weights per active phenol group for DBTHQ. Thus, the ranking of the efficiency of the radical scavengers corresponds to the variations in the phenol equivalent weight.

Table 6: Phenol equivalent weight for the applied free-radical scavengers.

The free-radical scavengers were unable to reduce the rate of cathodic delamination, when they were incorporated into the intermediate or topcoat. Thereby, the free-radical scavengers are solely effective when they are incorporated into the primer. This is because the radical scavengers must be present at the coating-steel interface where the free radicals are formed by the corrosion process. Furthermore, neither the free-radical scavengers nor the peroxide decomposer affected the strength of adhesion of intact coatings (18±2 MPa) or the free corrosion potential of the steel substrate (-745 mV SCE). This confirms that the mechanism by which free-radical scavengers reduce the rate of cathodic delamination is different from the mechanism of corrosion inhibitors and that the effect cannot be ascribed to

improvements in the strength of adhesion between the coating and the steel surface.

A hindered amine light stabilizer (Tinuvin 292 from Ciba), another specific type of antioxidant, was unable to reduce the rate of cathodic delamination when incorporated into the primer, intermediate or top-coat. The inability of hindered amine light stabilizers (HALS) to reduce the rate of cathodic delamination can be explained by the absence of ultraviolet radiation in the experimental facilities because HALS must be converted into stable nitroxyl radicals by ultraviolet radiation before they can scavenge free radicals. However, HALS will also be inefficient during natural outdoor exposure because multilayer systems typically consists of a primer and one or more intermediate coats and/or a topcoat, which are non- permeable to ultraviolet radiation and thereby prevents the conversion of HALS to nitroxyl radicals.

Secondary aromatic amines are efficient free-radical scavengers in the absence of ultraviolet radiation but will react with the epoxide groups in the epoxy resin and thereby be consumed. This was confirmed by monitoring the viscosity of a mixture with 50 wt% Epikote 828 and a secondary aromatic amine continuously by the Gardner-Holdt method (the kinematic viscosity increased from around 6.3-10^"3 m²s^_1 to 16- 10^"3 m²s^_1 within 3 hours). Thereby, secondary aromatic amines will not protect an epoxy coating from degradation by free radicals to the same extent as hindered phenols, when there is an excess of epoxide groups in the coating. Water-soluble and hydrophilic compounds will enhance the risk of blistering and subsequent coating failure significantly, therefore hydrophilic compounds such as ascorbic acid should not be incorporated into solvent-borne primers although they may be efficient free-radical scavengers. Concentration of free-radical scavenger

The rate of cathodic delamination of epoxy coatings modified with free-radical scavengers is highly dependent on the amount of free-radical scavengers as demonstrated in Table 7. The incorporation of 1-3 wt% free-radical scavengers results in significant improvements in the resistance towards cathodic delamination. When the amount of free-radical scavengers is increased further, up to 8 wt%, the resistance towards cathodic delamination is improved further. However, above 8 wt% the extent of cathodic delamination is increased. This may be the result of an increase in excess free volume caused by the incorporation of free-radical scavengers, which subsequently increases the permeability of the coating and the rate of cathodic delamination.

Table 7: Extent of cathodic delamination as function of the amount of free-radical scavenger (DTBHQ) of coatings based on a 50: 50 (w/w%) mixture of Epikote 828 and Epikote 1001 after 8 weeks of exposure to 0.5 M potassium chloride at free corrosion potential. Type of substrate

The formation of reactive intermediates during electrochemical reduction of oxygen is not restricted to substrates prepared from cold rolled steel. Specific examples of other types of metals, which under cathodic polarization will reduce oxygen to reactive intermediates include aluminium, copper, and stainless steel .

Table 8 shows the extent of cathodic delamination of a reference coating without free-radical scavengers and an identical coating modified with 3 wt% of free-radical scavenger (DBTHQ) for various types of metallic substrates under cathodic polarization.

Table 8: Extent of cathodic delamination of a reference coating based on a 50: 50 (w/w%) mixture of Epikote 828 and Epikote 1001 without free-radical scavengers and an identical coating containing 3 wt% radical scavengers (DBTHQ) for various types of substrates under cathodic polarization at -1050 mV (SCE). All specimens were immersed in 0.5 M sodium chloride for 9 weeks.^* Polarized to -1200 mV (SCE).

Although there are significant variations in ability of free-radical scavengers to reduce the extent of cathodic delamination for the different types of substrates, it is clear that incorporation of free-radical scavengers reduce the extent of cathodic delamination on the applied types of substrates. This confirms that the formation of free radicals is an integrated part of the delamination process of epoxy coatings on most metallic surfaces. The variations in the extent of cathodic delamination can be ascribed to the differences in the catalytic activity of the oxide layer on the surface of the metals.

Combinations of free-radical scavengers and adhesion promoters

The combined effects of the combination of 2 wt% epoxy functional silane (KBM-603; ex Shin-Etsu Chemicals) and amino functional silane (Dynasylan Glymo; ex Evonik Industries), respectively, and 2 wt% of a free-radical scavenger (DTBHQ) were tested during cathodic polarization at -1050 mV(SCE) and exposure to salt spray. In addition to the extent of cathodic delamination during cathodic polarization, Table 9 shows the extent of delamination and under-film corrosion during salt spray testing.

Table 9: Effect of combinations of adhesion promoters and free-radical scavengers in coatings based on a 50 : 50 (w/w%) mixture of Epikote 828 and Epikote 1001 on cathodic delamination under cathodic polarization at -1050 mV (SCE) for 9 weeks at 20°C as well as delamination and under-film corrosion curing salt spray testing for 26 weeks according to ISO 9227.

The addition of either an epoxy or an amino functional silane yields a reduction in the extent of cathodic delamination of specimens under cathodic polarization at -1050 mV (SCE) compared to the unmodified coating. The combination of anfree-radical scavenger and either an epoxy or an amino functional silane yields at least an additive effect in the resistance towards cathodic delamination compared to coatings modified with either an epoxy or an amino functional silane alone. This effect may be explained in terms of the different mechanisms by which free-radical scavengers and adhesion promoters protect the coating against degradation. The free-radical scavengers will, as previously discussed, inactivate the free radicals formed by the corrosion process. In contrast to free-radical scavengers, adhesion promoters do not inactivate free radicals. Adhesion promoters are covalently bonded to the steel surface and the silicon-oxygen bond have a high bond energy (443 kJ/mol) compared to a typical bond strength of 360 kJ/mol for the carbon-carbon bond of organic binders. Furthermore, the silanes are already oxidized, which prevents further oxidation by reactive intermediates.

The addition of free-radical scavengers gives a limited improvement in the resistance towards under-film corrosion and delamination during exposure to salt spray compared to the unmodified coating, probably because the mechanism of corrosion in sea water is different from atmospheric corrosion but this was not investigated further. The epoxy and amino functional silanes has no effect on the extent of under-film corrosion but reduce the extent of delamination during exposure to salt spray. Hence, it is clear that there is no direct correlation between delamination and under-film corrosion during salt spray testing.

Furthermore, the combination of a free-radical scavenger and an epoxy functional silane improves the resistance towards under-film corrosion compared to the coating containing the epoxy functional adhesion promoter. This suggests that free-radical scavengers may contribute to an improved resistance towards under-film corrosion.

Examples series B

Coating formulation

The coating compositions were prepared by adding 3% by solids volume of the additives shown in Table 10 to Hempadur 17630-12170 ex Hempel. The additives were mixed into the curing agent.

Table 10. Additives of the type free-radical scavenger (or other antioxidant (In italics)) included in the test compositions.

Preparation of test panels

Substrates prepared from cold rolled low alloy carbon steel (maximum 0.18 wt % carbon) were degreased with Hempel Light Clean, an alkaline detergent, and thoroughly rinsed with distilled water. Thereafter the substrates were abrasive blast cleaned with steel grit (type G9) to Sa 2¹/2 and medium roughness (R_y = 57 μηη). The coating was applied by airless spray at a pressure of 100 bars. The desired coating thickness was obtained by application of one layer of experimental epoxy coating with the different antioxidants plus one layer of standard Hempel's polyurethane coating 55210. The first coat was allowed to cure for 1 day at ambient temperature before the second coat was applied. A magnetic gauge instrument, Elcometer Model 355 Top, was used to measure the dry film thickness of the coatings, which was 200 μηη ± 25 μηη, representative of industrial coating thicknesses. The final curing time of the coating specimens was at least 7 days at ambient temperature.

More details about the specific panel preparation for each test in Test Method section.

Results and discussion

The effect of different phenolic free radical scavengers, a peroxide decomposer (Irgafos 168) and secondary amine (Irganox 5057) on the extent of cathodic delamination of epoxy coatings was tested. Summary of results are shown in Table 11.

Once again the tendency is that antioxidants are capable to reduce disbondment in epoxy resin coatings. In this experimental series no adhesion promoters was used. These results could be explained in terms of free radicals reduction at the steel-primer interphase when antioxidants are present according. It was not possible to measure the delamination effect of ascorbic acid due to prompt blistering in all the tests. The poor performance of ascorbic acid is most likely due to its high water solubility (330 g/L).

The series also demonstrated higher delamination tendency in CPT test of coating Example 7 (Vitamin E), and coating Example 8 (Beta Carotene) compared the others additives. Results from Example series B are in accordance with the results obtained in Example series A. These results can be explained using the same principles and protection mechanism detailed in Example series A.

Test Results

Coating

Antioxidant CPT KCI SST SW IMS BBT system CPT

(mm) (mm) DLam Creep DLam Creep

Irganox

Example 1 18.4 19.6 16.1 1.4 18.7 0.0 0

1135 Ciba

2,5-di-tert-

Example 2 butylhydro- 19.2 19.2 16.6 1.4 18.8 0.0 0 quinone

Irganox

Example 3 15.0 17.4 12.6 1.5 17.3 0.0 0

1010

4-Tert-butyl

Example 4 15.0 14.5 13.7 1.1 18.5 0.0 0 chatecol

Tert-butyl

Example 5 hydro- 16.8 15.9 18.2 0.9 19.4 0.0 0 quinone

Example 6 Irganox

19.5 20.2 15.0 1.8 19.5 0.0 0 (ref.) 5057 Ciba

Vitamin E/a-

Example 7 18.6 19.7 18.4 1.7 17.6 0.0 0 tocopherol

Example 8 Beta

19.6 22.3 14.9 1.7 21.6 0.0 0 (ref.) carotene

Vitamin C

Example 9

(ascorbic nd. nd. nd. nd. nd. nd. 5S4 (ref.)

acid)

Example 10

Irgafos 168 18.7 21.4 13.8 1.3 20.1 0.0 0 (ref.)

Reference None 22.4 21.9 15.4 1.0 20.9 0.0 0

Table 11 : CPT: Cathodic protection test, ISO 15711 (see above); CPT KCL: Cathodic protection test in KCL, ISO 15711 (see above); SST: Salt Spray Test, ISO 9227 (see above); SW IMS: Sea water immersion test, ISO 2812-2 (see above); BBT: Blister Box Test, ISO 6270-1 (see above); DLam : delamination; and Creep: Rust creep.

Claims

1. A method of applying a liquid epoxy-based anti -corrosive primer composition

comprising

- an epoxy-based primer system comprises one or more epoxy resins and one or more curing agents, and

- one or more free-radical scavengers in an amount of 0.5-20 % by weight, said free- radical scavengers being selected from a. phenols which are substituted with one or more groups selected from i. Ci-6-alkyl optionally substituted with a substituent selected from

phenyl, Ci-₂₄-alkyloxycarbonyl, Ci-₂₄-alkylcarbonyloxy, Ci-₂₄- alkylaminocarbonyl, Ci-24-alkylcarbonylamino, di(Ci-₆-alkyl)phosphono- oxy, Ci-24-alkoxy, and Ci-₂₄-alkylthio; ii. phenyl optionally substituted with one or more Ci-₆-alkyl; iii. hydroxy; Ci-₆-alkoxy; iv. Ci-6-alkylthio; v. amino; Ci-6-alkylamino; and di(Ci-6-alkyl)amino; with the proviso that the phenol is substituted with at least one group selected from optionally substituted Ci-₆-alkyl, and with the further proviso that if the phenol is exclusive substituted with optionally substituted Ci-₆-alkyl, at least one C₂-6-alkyl is present in the ortho position relative to the phenol functionality; and b. phenol multimers of the formula Z(Y)_n, wherein n is 2, 3, 4 or 5; and wherein Z is a central scaffold having a valence of n and having covalently bonded thereto n phenol moieties, Y, each phenol moiety, Y, independently being optionally substituted with one or more groups selected from i . Ci-6-al kyl optionally substituted with a substituent selected from phenyl, Ci-24-al kyloxycarbonyl, Ci-₂₄-alkylcarbonyloxy, Ci-₂₄- alkylaminocarbonyl, Ci-24-alkylcarbonylamino, di(Ci-₆-alkyl)phosphono- oxy, Ci-24-alkoxy, and Ci-₂₄-alkylthio; ii . phenyl optionally substituted with one or more Ci-₆-alkyl ; iii . hydroxy; Ci-₆-al koxy; iv. Ci-6-al kylthio; v. amino; Ci-6-alkylamino; and di(Ci-6-alkyl)amino with the proviso that if none of the phenol moieties are substituted, then the scaffold is not optionally substituted methylene; said method comprising the steps of:

(1) preparing the primer composition by mixing a component comprising the one or more epoxy resins and a component comprising the one or more curing agents, and

(2) applying said primer composition to at least at part of the surface of a metal substrate by spraying .

2. The method according to claim 1, wherein the free-radical scavenger is (a) a phenol .

3. The method according to claim 2, wherein the phenol is substituted with at least one tert-butyl group.

4. The method according to claim 1, wherein the free-radical scavenger is (b) a phenol multimer.

5. The method according to claim 4, wherein the central scaffold, Z, is selected from Ci-6-al kylene, -0-, -S-, -N < , l,3,5-trimethylene-2,4,6-tri-tert-butylbenzene, 1,3,5- trimethylene-2,4,6-tri-tert-butylbenzene, and pentaerythritol tetrakis(3-propionate), and -CH2CH₂C(=0)-NH-NH-C(=0)CH₂CH2- .

6. The method according to claim 5, wherein the central scaffold, Z, is selected from Ci-6-al kyl - l, l-ene, such as methylene, ethyl - l, l-ene, propyl- l, l-ene, propyl -2,2-ene, butyl - l, l-ene, and 2-methyl -propyl - l, l-ene.

7. The method according to claim 5, wherein the central scaffold, Z, is selected from -0-, -S-, and -N < .

8. The method according to any one of the preceding claims, wherein at least one

substituent is ortho relative to said phenol functionality.

9. The method according to any one of the preceding claims, wherein the phenol

equivalent weight of the compound is less than 400 g/mol, such as less than 350 g/mol .

10. The method according to any one of the preceding claims, wherein the one or more free-radical scavenger are present in an amount of 1.0- 15 % by weight.

11. The method according to any one of the preceding claims, wherein each of the one or more free-radical scavengers have a solubility in water of less than 0.2 g/mL at 25 °C, in particular less than 0.1 g/mL at 25 °C.

12. The method according to any one of the preceding claims, wherein the composition further comprises an adhesion promoter of the silane type.

13. A coating system on a metal surface, comprising coat of an epoxy-based primer

composition (i .e. a primer coat) on at least a part of the metal surface, and a coat of a second coating composition (e.g . a top-coat) on at least a part of the primer coat, wherein said primer composition is applied as defined in any one of the preceding claims.

14. The coating system according to claim 13, wherein the coat of the second coating composition represents a non-transparent coat.

15. An article coated with a primer composition according to the method of any one of the claims 1- 12.